A WLAN (Wireless Local Area Network) system works on an unlicensed spectrum; therefore, other users may share the spectrum on a channel. If multiple users send data at the same time, the data may interfere with each other, causing a collision. The WLAN system uses a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) mode to avoid a collision. Carrier sense CS refers to that any device connected to the channel first senses the channel before preparing to send data. The data may be sent only when it is confirmed that the channel is idle. Multiple access MA refers to that multiple devices may access a channel at the same time, and a data frame sent by one device may be received by multiple devices. A CSMA/CA mechanism resolves a collision problem by means of active collision avoidance instead of passive detection, which can satisfy those demands for which it is not easy to accurately detect whether a collision occurs. A working manner of CSMA is: when a device prepares to send a data frame and senses that a channel is idle, if the device further waits for a random time in which the channel is still idle, the device sends the data frame. However, there is a problem about a hidden node in a WLAN network; and when an access point AP1 sends data to a STA (Station), a station or an access point that cannot sense the access point AP1 may think that the channel is idle and send the data, which interferes with the receiving device STA. Such a problem about a hidden node cannot be resolved by using a CSMA/CA protocol.
At present, for the problem about a hidden node, the WLAN system protects transmission by using an RTS/CTS (Request to Send/Clear to Send) protocol, where a channel is reserved before a station or an access point sends data. The RTS/CTS protocol specifies: the access point AP1 first sends an RTS frame before sending the data, and the STA responds with a CTS frame after receiving the RTS frame. Each of other APs (Access Point) or STAs that receive the RTS or CTS frame sets a NAV (Network Allocation Vector) according to an indication of the received RTS or CTS frame. Within a time corresponding to the NAV, such AP or STA cannot send data on the corresponding channel. After sending the RTS frame and receiving the CTS frame returned by the STA, the AP1 obtains a sending opportunity, and if the AP1 sends data to the STA within this time period, the AP1 is not interfered by a surrounding AP or STA.
However, when APs and STAs are intensively deployed in the WLAN system, a collision probability of RTS frames greatly increases, and a contention window length after a collision doubles; therefore, not only system overheads are relatively high, but also a reduction in system efficiency is caused.